1. Field of the Invention
This invention relates to a scanning circuit for an image device and a driving method for the scanning circuit, and more particularly to a scanning circuit used as a peripheral circuit for a liquid crystal display, a contact-type image sensor, a liquid crystal shutter and so forth, and a driving method for driving the scanning circuit.
2. Description of the Related Art
Conventionally, in order to minimize, reduce in cost and achieve a high reliability of a liquid crystal display, a contact-type image sensor, a liquid crystal shutter and so forth, a thin film transistor driving circuit which is used as a peripheral circuit for those apparatus is integrated on them. The manufacturing method is adopted based on the concept that, where a peripheral driving circuit is provided on the same substrate on which picture element electrodes of a liquid crystal display, a contact-type image sensor, a liquid crystal shutter or the like are provided, the number of connection terminals and the number of external driver integrated circuits (ICs) are decreased remarkably and the problem in reliability which arises from a limitation in the large area and high density bonding step can be solved.
Usually, a scanning circuit which is used as a peripheral circuit for a liquid crystal display, a contact-type image sensor, a liquid crystal shutter or the like consists of a shift register and an output buffer. For example, in an active matrix liquid crystal display, the scanning circuit serves as a vertical driving circuit or a circuit for scanning sample and hold switches in a horizontal driving circuit and makes an important component which forms a thin film transistor driving circuit described above.
In a liquid crystal projector which is being spread as a large screen projection type display in recent years, an image for one of three liquid crystal light valves corresponding to the three primary colors of red, green and blue must be reversed by a mirror due to a difference among the number of reflections of light passing though the liquid crystal light valves. In order to achieve such mirror reversal, either the scanning direction of a vertical scanning circuit is reversed or the liquid crystal light valve is rotated by 180 degrees and the scanning direction of a horizontal scanning circuit is reversed. To this end, a bidirectional scanning circuit in which data can be switchably transferred leftwardly or rightwardly is required.
FIG. 4 shows the schematic diagram of a conventional bidirectional scanning circuit. Referring to FIG. 4, the conventional bidirectional scanning circuit has an input terminal STR to which a right shift start pulse signal is inputted and another input terminal STL to which a left shift start pulse signal is inputted. The conventional bidirectional scanning circuit includes N (N is a positive integral number) selection circuits 401-1 to 401-N, N shift registers 405-1 to 405-N of the half-bit configuration corresponding to the N selection circuits 401-1 to 401-N and having a function of delaying and transferring a pulse signal, and N output buffer circuits 406-1 to 406-N for outputting the outputs of the shift registers 405-1 to 405-N as outputs OUT1 to OUT(N), respectively. Each selection circuit, 401-1 to 401-N, consists of a pair of AND circuits 402 and 403 and an OR circuit 404. Meanwhile, each output buffer circuit, 406-1 to 406-N, consists of a pair of invertors 407 and 408.
FIGS. 5 and 6 illustrate operation timings of the conventional bidirectional scanning circuit shown in FIG. 4. The curves (a), (b), (c), (d), (e), (f), (g), (h), (i) and (j) of FIG. 5 show waveforms of different signals in the bidirectional scanning circuit of FIG. 4, when a pulse signal is transferred in the rightward direction from the left end of the bidirectional scanning circuit in FIG. 4, while the curves (a), (b), (c), (d), (e), (f), (g), (h), (i) and (J) of FIG. 6 show waveforms of such signals, when a pulse signal is transferred in the leftward direction from the right end of the bidirectional scanning circuit in FIG. 4. Operation of the conventional bidirectional scanning circuit will be described below with reference to FIGS. 4, 5 and 6.
In rightward shifting wherein a right shift start pulse is inputted to the input terminal STR and transferred in the rightward direction from the left end of the bidirectional scanning circuit in FIG. 4, the other input terminal STL is set to an open state. The right shift start pulse from the input terminal STR is inputted to the AND circuit 403 included in the selection circuit 401-1. Meanwhile, an input signal A to be inputted to the other input terminal of the AND circuit 403 is set to a high level and another input signal B to be inputted to an input terminal of the AND circuit 402 is set to a low level. Due to the input level settings to the AND circuit 402 and the AND circuit 403 Just described, the AND circuit 403 to which the input signal A of the high level is inputted is selected. This similarly applies to the AND circuits 402 and 403 included in the selection circuits 401-2 to 401-N, and the AND circuits 403 are selected in response to the input signal A of the high level, thereby forming a rightwardly shifting scanning circuit.
The right shift start pulse inputted from the input terminal STR is inputted to the shift register 405-1 by way of the AND circuit 403 and the OR circuit 404, and a pair of clock signals .phi.1 and .phi.2 (which is inverted clock signal of .phi.1 are inputted also to the shift register 405-1. Thus, the timing of a signal to be outputted from the shift register 405-1 is controlled by the clock signals .phi.1 and .phi.2, and a scanning pulse signal is outputted as an output signal OUT1 by way of the output buffer circuit 406-1. The signal outputted from the shift register 405-1 is inputted to the AND circuit 403 included in the selection circuit 401-2 at the next stage so that it is inputted to the shift register 405-2 by way of the AND circuit 403 and the OR circuit 404. The operation of the shift register 405-2 then is quite similar to the operation of the shift register 405-1 described above in that the timing of a signal to be outputted from the shift register 405-2 is controlled by the clock signals .phi.1 and .phi.2 and a scanning pulse signal is outputted as an output signal OUT2 from the shift register 405-2 by way of the output buffer circuit 406-2. The scanning pulse signal is simultaneously inputted also to the AND circuit 403 included in the selection circuit 401-3 at the next stage. Thereafter, a scanning pulse signal is outputted as an output signal OUT(N-1) from the N-1th output buffer circuit 406-(N-1) in a similar manner as described above, and a scanning pulse signal is outputted as an output signal OUT(N) from the Nth output buffer circuit 406-N. In this manner, successively shifted scanning pulse signals are outputted as output signals OUT1, . . . , OUT(N-1) and OUT(N) in this order (refer to FIG. 5).
On the other hand, in leftward shifting wherein a left shift start pulse is inputted to the input terminal STL and transferred in the leftward direction from the right end of the bidirectional scanning circuit in FIG. 4, the other input terminal STR is set to an open state. The left shift start pulse from the input terminal STL is inputted to the AND circuit 402 included in the selection circuit 401-N. Meanwhile, the input signal B to be inputted to the other input terminal of the AND circuit 402 is set to the high level and the input signal A to be inputted to an input terminal of the AND circuit 403 is set to the low level. Consequently, the AND circuit 402 to which the input signal B of the high level is inputted is selected. This similarly applies to the AND circuits 402 and 403 included in the selection circuits 401-1 to 401-(N-1), and the AND circuits 402 are selected in response to the input signal B of the high level so that a leftwardly shifting scanning circuit is formed.
The left shift start pulse signal inputted from the input terminal STL is inputted to the shift register 405-N by way of the AND circuit 402 and the OR circuit 404 included in the selection circuit 401-N, and the pair of clock signals .phi.1 and .phi.2 (which is inverted clock signal of .phi.1) are inputted to the shift register 405-N, and the timing of a signal to be outputted from the shift register 405-N is controlled by the clock signals .phi.1 and .phi.2. A scanning pulse signal is outputted as an output signal OUT(N) by way of the output buffer circuit 406-N. The signal outputted from the shift register 405-N is inputted to the AND circuit 402 included in the selection circuit 401-(N-1) at the next stage so that it is inputted to the shift register 405-(N-1) by way of the AND circuit 402 and the OR circuit 404. The operation of the shift register 405-(N-1) then is quite similar to the operation of the shift register 405-N described above in that the timing of a signal to be outputted from the shift register 405-(N-1) is controlled by the clock signals .phi.1 and .phi.2 and a scanning pulse signal is outputted as an output signal OUT(N-1) from the shift register 405-(N-1) by way of the output buffer circuit 406-(N-1). Thereafter, a scanning pulse signal is outputted as an output signal OUT3 from the output buffer circuit 406-3 in a similar manner as described above, and scanning pulse signals are outputted as output signals OUT2 and OUT1 from the output buffer circuits 406-2 and 406-1, respectively. In this manner, successively shifted scanning pulse signals are outputted as output signals OUT(N), OUT(N-1), . . . and OUT1 in this order (refer to FIG. 6).
In the conventional bidirectional scanning circuit described above, since N stages of selection circuits are provided, corresponding additional lines must be provided. Such additional lines require a corresponding large circuit occupation area and provide a corresponding large capacitance, and consequently, it is difficult to minimize the bidirectional scanning circuit and increase the operation speed of the bidirectional scanning circuit. Accordingly, the conventional bidirectional scanning circuit is disadvantageous in that it cannot be applied to a liquid crystal display, a contact-type image sensor or a like apparatus which requires a high speed operation and a high resolution.
Further, since the conventional bidirectional scanning circuit has a large circuit occupation area, the yield in manufacture of scanning circuits is low. Furthermore, since the scanning circuit consists of shift registers connected in series, if only one of the shift registers fails, a scanning signal cannot be transferred regularly to circuits connected to those shift registers following the failed shift register. In a two-dimensional image apparatus such as a liquid crystal display, such incomplete transfer of a scanning signal causes a fatal image defect. Since the defect appears even where a picture element array section has no defect, there is a drawback in that the defect itself of the scanning circuit makes a factor which deteriorates the yield of devices.